Surgical teams leaving tools or other foreign bodies inside pediatric patients is a serious medical error that is most common in gynecological surgeries adds to the cost and duration of a child’s hospital stay, but not to mortality rates.

That’s the conclusion from researchers at Johns Hopkins University, who studied a national database of 1,946,831 operations in children over 17 years, and in 36 states, and found 413 incidents in which sponges or other surgical instruments were unintentionally left, requiring follow-up procedures to remove them.